The present invention relates generally to a scheme by which a furnace used to melt process alloys at high temperature employing Plasma Arc Melting (PAM) or Electron Beam Melting (EBM) can be visually observed. More particularly, it relates to a scheme for providing and preserving an optical path into the furnace chamber while the melt processing is in progress and permitting viewing of almost the entire interior of the furnace chamber by a clean surfaced mirror.
It has been found that particulate matter which is generated from the melt surface of PAM or EBM specimens tends to deposit on viewports and associated apparatus used for monitoring the melt processing of these specimens either visually or optically. Such deposits of particulate material interfere with visual observation used for process monitoring and also interferes with quantitative infrared temperature measurements used for process control as well as with other optical techniques, such as mirror viewing, used either for process control or monitoring of the processing taking place within the furnace. For example, visual observation of the melt pool level is sometimes necessary in order to monitor the level for processing purposes.
Prior approaches employed for dealing with optical path window fowling of this nature have included a variety of mechanical and optical accessories. Some of these have included, for example, movable films, wipers, brushes, pinhole lenses, mirrors, and a variety of shutters. However, none of these means has been satisfactory on an extended use basis.
For example, with regard to movable films, these films have not proven reliable and when the film fails it presents a possibility of introducing foreign material into the melt chamber. Further, such films are not always suitable either in terms of optical quality or in terms of spectral response where instrumented observation is employed.
In another mode, wipers as well as brushes are employed periodically to clean the interior surface of a viewport or of an associated mirror. However, this leaves the optical properties of these windows and mirrors time dependent and, for the most part, unpredictable. Further, from such wiping or brushing, the deposits are freed from the window or mirror surface but ma be caused to fall into the chamber and into the melt to cause contamination of the melt.
In another mode, shutters are employed both of a manual and of a motorized character. However, the shutters only reduce the rate of deposition and do not preclude the deposition from occurring. Further, shutters introduce moving parts and seals and this introduction can compromise the furnace atmosphere. The use of shutters means that the presence or absence of the window or mirror deposits remains time dependent and, further, there is no continuous viewing when the shutter mechanism is in operation.
Conventional gas purged windows or mirrors have not been effective in preventing deposition. This is because the conventional gas purge scheme involves the high flow of gas past a window and the high flow induces eddy currents which cause a backflow of the contaminant particulate matter thus resulting in an irregular deposition of material on the window interior. The nonuniform deposition actually increases the problem of using such viewports.
A successful window is described in the copending application Serial No.390,052 filed Aug. 7, 1989. This mechanism requires a gas flow and such flow is suitable for many furnacing operations. However, for electron beam melting a good vacuum must be maintained in the chamber. Such a vacuum cannot be maintained where purge gas is used or is needed. Further, the mean free path of gas particles is very large at low pressure and a very long interception tube may be required where a viewport or mirror is used in connection with a gas purge as described in the copending application.
Some of the problems of keeping a mirror surface free of contamination from deposited vaporous or particulate matter are particularly acute. The real advantages of use of a mirrored surface in connection with a viewport is that it permits much broader zones within the furnace enclosure to be viewed. In other words, the optical path available to a viewport is quite limited to what lies directly ahead of the transparent element through which the viewing is done. However, it is frequently desirable to see into portions of the furnace chamber which are not optically aligned with the viewport. Efforts to provide reflective surfaces have suffered from the same problems as afflict the viewports themselves. Occlusion of solids, and particularly finely divided solids which contain significant oxygen levels clouds the surface and presents need for accessory equipment or operations as described above to keep them clean.
Moreover, the viewport is usually remote from the point where high levels of heat are applied and the fine particulate matter is generated. A mirror surface is likely to be closer to the source of contaminant and for this reason is more likely to become contaminated.
We have now devised a means for maintaining the optical path through a viewport in an apparatus which employs high intensity heating such as heating through PAM or through EBM or similar high intensity means and where this high intensity heating results in the formation of particulate matter which develops into a cloud or fog within a furnacing chamber.
The apparatus of the present invention is particularly suitable for use in connection with the high intensity top heating of a metal bath as by electron beam or plasma heating. Such high intensity surface heating causes generation of substantial amounts of metal vapor and/or very fine particles. It is toward the reduction in fowling and contamination of optical paths within the furnace chamber because of vapors and fine particles toward which the subject invention is directed.